Inductance core having low negative temperature coefficient of inductance and method of making it



Patented Feb. 11, 1941 UNITED STATES INDUCTANCE CORE HAVING LOW NEGATIVETEMPERATURE COEFFICIENT OF INDUCT- ANCE AND METHOD OF MAKING 1'1 PaulGottschalt, Beriin-Spandan, Germany, as-

signor to Siemens & Halske Aktiengesellschaft, Siemensstadt, nearBerlin, Germany, a corporation of Germany No Drawing. Application April18, 1938, Serial No. 204,926. In Germany April 17, 1937 lclalms.

(Granted under the provisions of sec. 14, act of March 2, 1927; 357 0.G. 5)

The invention relates to a method for producing coil cores ofmagnetizable material.

, For numerous purposes in the field of electrical communication,inductance coils are required, the

5 inductance of which must not change more than to an extremely slightdegree for long periods of time (T). In addition, it is often necessaryto make exacting demands with respect to the temperature coemcient ofthe inductance (L). There are cases where as small a temperaturecoefllcient as possible is desired, for instance, m 1X degree but moreoften it is required that the temperature coefiicient come as near aspossible to a certain pregiven value, for instance, =-3xl0- degree Sucha given value, usually negative, of the temperature coeflicient isrequired especially in such cases where the coils are to be arranged inoscillating circuits, band filters and similar connections withcondenser circuits, and where the natural frequencies of thesecoil-condensercircuits are supposed to have a high constancy and aslight dependence on temperature. These natural frequencies in depend,in general, on the inductance (L) of the coil and the capacity (C) ofthe condenser in accordance with the relation 1 21 /110 7 The oftennecessary high constancy of the natural frequency I0 is obviouslyattained if- (I) L and C at any maintained temperature T remainunchanged in time within the temperature range met with in regularservice, and if (2) Within this entire range of temperature thetemperature coeilicient,

my ""'L or of the coils with reversed signs corresponds to that of thetemperature coefliclent ..Q "0 OT of the condenser, viz., if therelation 55 measures, it is usually endeavoured to satisfy the relation(2) by arrangements at the coil, which offers more technicalpossibilities than is the case with the condenser.

The temperature coeflicient of the condensers mostly in use in technicalengineering is in general positive in accordance with the linearcoefficient of expansion of the material employed, so that in most casesa negative temperature coefflcient of the coil must be required. Forinstance, the frequently used mica condenser in ambient air may have thecoefficient a=+3X1U degreeso that for the coils to be connected withsuch condenser the coeflicient a=3X10" degree is prescribed.

However, the usual coils employed in electrical 15 communication, inparticular the coils without ferromagnetic core, such as are common forvery high frequencies (short waves), show in the great majority of casesa positive temperature coefflcient 11 With such coils, therefore,special go technical measures are necessary for obtaining a certainpre-given negative temperature coefficient, for instance, -3X10- degreeA number of such measures are known, yet they have considerabledrawbacks, or are only suitable for 25 a greatly limited special use ofthe coils.

The invention does away with these disadvantages. According to theinvention the magnet core of inductance coils designed to meet theaforementioned requirements, is produced by 30 mixing ferromagneticpowder and an insulating substance, and/or several ferromagnetic kindsof powder with differing temperature coeflicients, and/or severalthermoplastic binding materials, while maintaining a mutual proportionresulting 35 in a total temperature coefficient of pre-given smallvalue, and by squirting the thus prepared mixture to obtain the desiredshape of the core.

According to a preferred embodiment of my invention, the ferromagneticportion of the mixture 0 to be squirted consists of carbonyl iron powderor contains carbonyl iron powder as the major component, i. e. in anamount of at least as compared with the total weight of theferromagnetic portion. Especially favorable results are obtained 5 with50 to 80% carbonyl iron powder and 50 to 20% of an iron-nickel alloy ofhigh magnetic permeability. The latter alloy may consist of about 78.5%nickel, the remainder iron. The insulating binder to be mixed with theferromagnetic por- 50 tion may consist of thermoplastic materials,preferably polystyrol and the like. The binder is to be used in aproportion of 1/10 to 5/10 as compared with the weight of theferromagnetic portion. If normal polystyrol is employed, proportionsbetween 2/10 and 5/10 have proved favorable, while when polystyrol ofhigher molecular weight is used, a proportion between 1/10 and 3/10 ispreferable.

For elucidating the invention, some comparative examples of differentlyproduced mass cores commonly used for Pupin coils, and the dependence ofthe inductance on the temperature for a number of temperature changesare referred to in the following. As Example No. 1, a core made ofcarbonyl iron powder and insulated by organic substances is taken inview; and as Example N0. 2 a core made of ferro-nickel alloy powder,which has been insulated with glow-resistant inorganc substances, andafter the pressing has been annealed in known manner at about 500.

A mass core according to Example No. 1, is unsuited for coils of highconstancy from the outset, owing to its considerable want ofreproductive capacity, i. e. the material and permanent alterationsafter repeated changes of temperature. The temperature coefficient,while. negative, is too large (a =17X10 degree In the case of ExampleNo. 2, the want of reproductive capacity is considerably less, but thetemperature coefficient is highly positive It has further been found,that the constancy in time of all those cores, which have been producedby mechanical pressing together of the insulated powder, cannot beincreased at all, or only under considerable difliculties, to the extentfrequently required.

While it is known to produce mass cores not by purely mechanicalpressing-together, but by die casting, such mass cores do not offhandyield the low magnitudes of the temperature coefficient required in eachcase. It has however been foundand this recognition forms a basis of thepresent invention-that with cores produced by the die casting method infact a better constancy in time can be attained, than with the coresproduced up to now principally by the pressing method.

As Example No. 3, a magnet coil may be considered, having a coresquirted in known manner of carbonyl iron. Such a coil shows fewerchanges in its inductance with repeated changes of temperature, i. e.the want of reproductive capacity is greatly reduced as compared withthat according to Example No. 1. The temperature coefficient is, infact, negative, as generally required, but not to the desired amount ofabout As compared with the foregoing three types of cores, a materialadvance with respect to constancy in time and reproductive capacity,together with a low magnitude of the temperature coefficient, isobtained if the cores are produced according to the invention byemploying certain mixture proportions and producing the cores bysquirting. An inductance coil having such a core, shows, for instance, atemperature coefficient of the desired value d=3 10- degree and sufferspractically no changes of this value during a period of several months,i. e. such changes remain materially below :lx 10 A method of producingsuch cores is the following. 60% carbonyl iron powder is mixed with 40%powder of a ferro-nickel alloy containing 78.5% nickel contents. Thispowder mixture is pre-insulated in the usual manner with a thin layer ofinsulating material, then mixed with 15 weight units of polystyrolpowder and eventually formed in known manner into annular cores bysquirting. By altering the mixture proportions, i. e. of the iron andalloy mixture or of the metal and polystyrol mixture or of bothproportions, the temperature coefficient can be changed at will withinlimits wide enough to allow satisfying all conditions actuallyoccurringin practice. In each particular case, the most favorablemixture proportions may be computed by an interpolation based upon themeasured temperature coefiicie'nts of known cores with similar mixtureproportions, or may be newly determined by measurings of cores withdiffering mixture proportions.

In order to obtain as highpermeabilities as possible, it is preferableto choose as high a share of ferromagnetic powder as possible, as longas this does not noticeably impair the capability of being squirted, andto vary the magnitude of the temperature coefficient by correspondinglyvarying the mixture proportion between two differing kinds offerromagnetic powder.

But there exist also fields of application, in particular with regard tocoils for very high working frequencies, for instance .short waves,where very slight permeabilities are desired owing to the unavoidablelosses in the iron core increasing with the frequency. In cases of thisnature the required temperature coefficient can be obtained by using agreater share of insulating material, or also, without the use ofdifferent kinds of ferromagnetic powder, merely by exactly maintaining amixture proportion between a ferromagnetic powder and a thermoplasticmass, which proportion may be determined in a given case, for instance,by some tests made with cores of different mixture proportions.

Surprisingly favorable results are obtained not only by the heretoforecustomary polystyrol, but by similar thermoplastic masses, which,however, have a higher thermal stability. Squirted mass cores ofcarbonyl iron powder and a polystyrol, having an extremely high meanmolecular weight of, for instance, more than 100,000, resulted in veryslight temperature coefficients and a good constancy in time. Coils withannular cores made of a mixture with 15 weight units of this polystyrolto units of iron showed by measurements the small value a =4 10- degreeand coils with other core shapes of the same core substance showedvalues between. a =2X10 degree and .=i, =5 10- degree Further, thetemperature coefficient may also be affected in the manner desired bythe simultaneous use of different thermoplastic masses in certainmixture proportions, for instance, by mixing a part of the saidpolystyrol, having a high molecular weight, with normal polystyrol. Theuse of polyvinyl-carbazal has also yielded very good results.

Physical examinations conducted in order to find an explanation of thecauses for the improvement obtained by the invention, have shown thatthe better constancy in time and the improved reproductive capacity ofthe cores is due to the thorough bond of the iron powder with theinsulating substance effected by the squirting method and that theeffect of the mixture proportion on the temperature coefficient is duein particular to the thermic coefficient of expansion.

What is claimed is:

1. The method of producing magnetizable cores for inductance coilshaving a temperature coefficient of inductance of pre-given negativevalue and high constancy in time, comprising the steps of mixing apowdered ferromagnetic material consisting of at least 50% of carbonyliron powder with an insulating binder composed of polystyrol powder ofnormal molecular weight and polystyrol powder of a higher mean molecularweight, said powders being empoyed in a mutual proportion of weightdesigned for resulting in a temperature coeflicient of pre-givennegative value, and shaping said mixture by squirting.

2. A magnet .core consisting of a squirted mixture of a ferromagneticmass composed o! 50 to 80% carbonyl iron powder and 50 to 20% powder ofan iron-nickel alloy with about 78.5% nickel with an insulating bindercontaining polystyrol substances oi diflerent molecular weight, andcharacterized by alow negative temperature coeflicient oi inductance andhigh constancy in time of said coeflicient.

3. A magnet core consisting of a squirted mixture consisting of aferromagnetic-mass composed of carbonyl iron powder and a powderedironnickel alloy of high magnetic permeability, and of an insulatingthermoplastic binder consisting substantially of polystyrol, T; to 1% inweight as compared with the weight of said ferromagnetic mass, andcharacterized by a low negative temperature coeflicient of inductanceand high constancy in time of said coeflicient.

4. A magnet core consisting of a squirted mixture of ferromagnetic massand an insulatin binder, said ferromagnetic mass being composed of 50 to80% carbonyl iron powder and 50 to 20% powder of aniron-nickel alloywith about 78.5% nickel, and said binder consisting of polystyrol andweighing 1% to /2 of the weight of said ferromagnetic mass.

- PAUL GOTISCHALT.

said binder forming

